The localization of tRNA5Asn, tRNAHis, and tRNAAla genes from drosophila melanogaster by in situ hybridization to polytene salivary gland chromosomes

Secrets of secretion-How studies of the Drosophila salivary gland have informed our understanding of the cellular networks underlying secretory organ form and function

Secretory organs are critical for organismal survival. Yet, the transcriptional regulatory mechanisms governing their development and maintenance remain unclear for most model secretory organs. The Drosophila embryonic salivary gland (SG) remedies this deficiency as one of the few organs wherein direct connections from the expression of the early patterning genes to cell specification to organ architecture and functional specialization can be made. Few other models of secretion can be accorded this distinction. Studies from the past three decades have made enormous strides in parsing out the roles of distinct transcription factors (TFs) that direct major steps in furnishing this secretory organ. In the first step of specifying the salivary gland, the activity of the Hox factors Sex combs reduced, Extradenticle, and Homothorax activate expression of fork head (fkh), sage, and CrebA, which code for the major suite of TFs that carry forward the task of organ building and maintenance. Then, in the second key step of building the SG, the program for cell fate maintenance and morphogenesis is deployed. Fkh maintains the secretory cell fate by regulating its own expression and that of sage and CrebA. Fkh and Sage maintain secretory cell viability by actively blocking apoptotic cell death. Fkh, along with two other TFs, Hkb and Rib, also coordinates organ morphogenesis, transforming two plates of precursor cells on the embryo surface into elongated internalized epithelial tubes. Acquisition of functional specialization, the third key step, is mediated by CrebA and Fkh working in concert with Sage and yet another TF, Sens. CrebA directly upregulates expression of all of the components of the secretory machinery as well as other genes (e.g., Xbp1) necessary for managing the physiological stress that inexorably accompanies high secretory load. Secretory cargo specificity is controlled by Sage and Sens in collaboration with Fkh. Investigations have also uncovered roles for various signaling pathways, e.g., Dpp signaling, EGF signaling, GPCR signaling, and cytoskeletal signaling, and their interactions within the gene regulatory networks that specify, build, and specialize the SG. Collectively, studies of the SG have expanded our knowledge of secretory dynamics, cell polarity, and cytoskeletal mechanics in the context of organ development and function. Notably, the embryonic SG has made the singular contribution as a model system that revealed the core function of CrebA in scaling up secretory capacity, thus, serving as the pioneer system in which the conserved roles of the mammalian Creb3/3L-family orthologues were first discovered.

Structure and transcription of eukaryotic tRNA genes

The availability of cloned tRNA genes and a variety of eukaryotic in vitro transcription systems allowed rapid progress during the past few years in the characterization of signals in the DNA-controlling gene transcription and in the processing of the precurser RNAs formed. This will be the subject matter discussed in this review.

The genes coding for 4 snRNAs of Drosophila melanogaster: localization and determination of gene numbers

Four small nuclear RNAs (snRNAs) have been isolated from Drosophila melanogaster flies. They have been characterized by base analysis, fingerprinting, and injection into Axolotl oocytes. The size of the molecules and the modified base composition suggest that the following correlations can be made: snRNA1 approximately U2-snRNA; snRNA2 approximately U3-snRNA; snRNA3 approximately U4-snRNA; snRNA4 approximately U6-snRNA. The snRNAs injected into Axolotl oocytes move into the nuclei, where they are protected from degradation. The genes coding for these snRNAs have been localized by "in situ" hybridization of 125-I-snRNAs to salivary gland chromosomes. Most of the snRNAs hybridize to different regions of the genome: snRNA1 to the cytological regions 39B and 40AB; snRNA2 to 22A, 82E, and 95C; snRNA3 to 14B, 23D, 34A, 35EF, 39B, and 63A; snRNA4 to 96A. The estimated gene numbers (Southern-blot analysis) are: snRNA1:3; snRNA2:7; snRNA3:7; snRNA4:1-3. The gene numbers correspond to the number of sites labeled on the polytene salivary gland chromosomes.

Localization of tRNA genes of Drosophila melanogaster by in situ hybridization

Six purified tRNAs labeled with 125I by chemical or enzymatic methods were hybridized to polytene chromosomes of Drosophila melanogaster. The main chromosomal regions of hybridization wer: tRNAGly(GGA), 58A, 84C, and 90E; tRNALeu(2), 44E, 66B5-8, and 79F; tRNASer(2b), 86A, 88A9-12, and 94A6-8; tRNAThr(3), 47F and 87B; tRNAThr(4), 93A1-2; and tRNATyr(1 gamma), 19F, 22F-23A, 41, 50C1-4 and 85A. At 50C the hybridization of tRNATyr(1 gamma) was polymorphic in the giant strains. When the hybridization of three valine isoacceptors studied previously was re-investigated, it was found that only one hybridization site, 90BC, was shared between tRNAVal(3b) and tRNAVal(4). tRNAVal(3a) did not have any sites in common with the other two.

Glutamate tRNA genes are adjacent to 5S RNA genes in Drosophila and reveal a conserved upstream sequence (the ACT-TA box)

In Drosophila melanogaster at least six transfer RNA genes are located adjacent to the 3' end of the 5S RNA gene cluster. Three of these have been sequenced and identified as coding for glutamate tRNA4. In the chromosome they are arranged as tandem repeats on the same DNA strand and transcribed in the same direction as is 5S DNA, towards the centromere. We have also identified a sequence, the ACT-TA box, that is highly conserved among the polymerase III transcribed genes. Usually the sequence is located at 37 +/- 8 base pairs upstream from the first nucleotide of the structural gene. A similar sequence is also observed upstream of yeast and silkworm tRNA genes and the mitochondrial tRNA genes of mouse and humans.

RNA-DNA hybridization analyses of tRNA-Val-3b in Drosophila melanogaster

Transfer RNA was extracted from 50-300 mg of adult flies and specifically labeled in vitro. The level of individual isoacceptors was quantitated by efficient annealing to Drosophila tRNA genes carried on recombinant DNA plasmids immobilized on nitrocellulose filters. The level of tRNAVal3b in the tRNA isolated from flies deficient in the major tRNAVal3b loci has been examined. The results show that deletion of the major tRNAVal3b loci resulted in a reduction of approximately 50% in the level of tRNAVal3b but did not produce the Minute phenotype; furthermore the effects of deficiencies at two loci were approximately additive.

The genes coding for tRNA Tyr of Drosophila melanogaster: localization of determination of the gene numbers

Transfer RNA(Tyr) (anticodon G psi A) was isolated from Drosophila melanogaster by means of Sepharose 4B, RPC-5, and polyacrylamide gel electrophoresis. The rRNA was iodinated in vitro with Na125 I and hybridized in situ to salivary gland chromosomes from Drosophila. The genes of rRNA(Tyr) were localized in eight regions of the genome by autoradiography. Restriction enzyme analysis of genomic DNA indicated that the haploid Drosophila genome codes for about 23 tRNA(Tyr) genes. The regions 22F and 85A each contain four to five tRNA(Tyr) genes, whereas the regions 28C, 41AB, 42A, 42E, and 56D each contain two to three tRNA(Tyr) genes.

The structure and function of tRNA genes of higher eukaryotes

The most recent findings concerning the structure and function of tRNA genes of higher eukaryotes are discussed in an exemplary way. The tRNA genes of higher organisms are either dispersed or clustered at different sites of the genome. Clusters contain tRNA genes oriented in both directions and on both strands of the DNA with spacers of various length inbetween. Some genes contain intervening sequences close to the 3' side of the anticodon. The primary transcription product possesses a 5' leader and a 3' trailer sequence which are removed by several maturation steps in a strict temporal and spacial order. Internal transcription control regions (promotors) are located at the 5' and 3' ends of the mature tRNA coding section of the tRNA gene. External sequences modulating the efficiency of the expression are present at the immediate 5' ends of the genes. Transfer RNA genes are located nonrandomly in the nucleosomes.